1. Field of the Invention
The present invention relates to a power conversion device such as a chopper or an inverter and, more particularly, to a power conversion device capable of preventing an insulated gate bipolar transistor from being broken by static electricity or the like.
2. Background Art
A transfer-mold power module (TPM), which is a large-current high-withstand-voltage power conversion device, is being used in intelligent power modules (IPMs) for motor vehicles because it is compact and has high reliability.
FIG. 22 is a circuit diagram showing a conventional power conversion device 1 and an external circuit 4. The power conversion device 1 incorporates an insulated gate bipolar transistor (IGBT) 2 and a free wheeling diode (FWD) 3 in a transfer-mold package. The external circuit 4 is provided outside the power conversion device 1. The gate, the current sense terminal and the emitter of the IGBT 2 are connected to the external circuit 4 via a G terminal 5, an Es terminal 6 and an Ec terminal 7, respectively. The collector and the emitter of the IGBT 2 are respectively connected to output terminals: a C terminal 8 and an E terminal 9. The free wheeling diode 3 is connected between the C terminal 8 and the E terminal 9.
The external circuit 4 has a function to drive the IGBT 2 and a function to protect the IGBT 2 from a large current caused by a short circuit for example. However, there is a problem that if the external circuit 4 has a function to protect the IGBT 2 from a large current, the delay in response to a large current is increased because of an increase in the length to the IGBT 2. There is also a problem that a malfunction occurs due to induction noise or the like.
To solve this problem, a power conversion device having, in the power conversion device 1 with the IGBT 2, a protective circuit 10 for protecting the IGBT 2 from a large current as shown in FIG. 23 has been proposed (see, for example, Japanese Patent Laid-Open No. 62-143450). This protective circuit 10 is a real time control (RTC) circuit having an NMOS transistor 11, a first resistor 12 and a first diode 13. The NMOS transistor 11 has its gate connected to the current sense terminal of the IGBT 2, its source connected to the emitter of the IGBT 2 and its drain connected to the gate of the IGBT 2 via the first diode 13. The first resistor 12 is connected between the current sense terminal and the emitter of the IGBT 2.
The NMOS transistor 11 is turned on when a large current flows through the current sense terminal of the IGBT 2 such that a threshold voltage is reached across the first resistor 12. The large current flowing through the IGBT 2 is thereby reduced or shutoff, thus preventing breakdown of the IGBT 2.
As described above, an RTC circuit is incorporated in the conventional power conversion device to prevent the insulated gate bipolar transistor from being broken by an excess current as well as to prevent a malfunction due to induction noise. In the conventional power conversion device, however, the insulated gate bipolar transistor cannot be prevented from being broken by static electricity generated during handling or testing.